Patent GB-778059-A (GB)
Title: Apparatus responsive to the departure of a member from a predetermined position
Inventor: HOLMES FRANK
IPC: G01B11/00
Filed: 1953-04-30
Abstract: 778,059. Automatic position control systems. HILGER & WATTS, Ltd. July 30, 1954 [April 30, 1953], No. 12022/53. Class 38 (4). [Also in Group XL (b)] Apparatus for automatically returning a member to a reference position comprises means for producing two groups of light beams each consisting of one or more beams, means movable with the member whereby the relative intensity of one group with respect to the other is made dependent on the position of the member relative to the reference position and means for directing the two groups alternately on a light detector whereby the relative intensity may be detected. An opaque masking strip 4 fixed to a machine member which is to be brought to a number of predetermined positions is movable across a fixed beam of light from source 2, each of the positions corresponding to centralization of a transparent slit in the strip with respect to the light beam. An image of lens 3 illuminated by the source is formed on the masking strip by lens 31 and when one of the slits is within this image an image of the slit is formed by lens 7 on the edge of beam-splitting prism 9 set on the beam axis so that the two beams 10, 11 are formed, the relative intensities of which depend on the relative positions of the slit and the prism edge. These beams, which are rendered parallel by double prisms 12, 13, are alternately blocked by sector plate 16 rotated by motor 18, so that coincident images of the source filament, the relative intensities of which correspond to the division of the slit image by prism 9 are alternately formed on the cathode 23 of photomultiplier 24 by lenses 19<SP>1</SP>, 20<SP>1</SP> and prisms 19, 20 respectively and prism 21. When the slit image is divided in half by the prism edge the intensities of the two images are the same so that a steady output current is obtained. If the slit deviates from the centralized position the output current has an alternating component the phase and amplitude of which depends on the sense and magnitude of the displacement. The output of the photo-multiplier developed across resistor R1 (Fig. 4) is amplified in valves 25, 26 and operates relay A when the average illumination is above a certain level. When the opaque parts of the masking strip 4 blocks the beam from lamp 2 the relay is inoperative and its contacts al short-circuit the input of amplifier 27. Contacts a2 apply, via resistors R3, R4, and capacitor C a reference signal of variable phase at the frequency of obturation of beams 10, 11, to amplifier 271 which feeds the control winding of a two-phase motor driving the machine member, the other winding being fed with a voltage of the same frequency. The reference signal may be generated by an electromagnetic pick-up co-operating with the sectors or with ferromagnetic parts fixed to the sectors or by a capacitor plate co-operating with the sectors. At the start of a series of machining or other operations the strip masks the beam and the phase of the reference signal is adj usted so that the motor drives the machine member until one of the slits falls well within the image of lens 3 when relay A is operated to remove the reference signal from amplifier 27<SP>1</SP> and to remove the short-circuit across the input of amplifier 27. The output of the photo-multiplier, amplified at 27, is thus fed to amplifier 27<SP>1</SP> in place of the reference signal so that the motor constrains to move the member towards the position where the slit is centrally disposed with reference to the prism edge, when the alternating output of the photo-multiplier falls to zero and the motor stops. Movement of the member to a position associated with the next slit is initiated by blocking the beam from source 2 or by shortcircuiting the input of amplifier 25 either manually or automatically in response to the end of some operation, e.g. a matching operation, whereupon the reference signal is applied to the motor and the cycle of operations repeated for the next slit. Alternatively the appropriate beam (10 in the illustration of Fig. 1) is blocked so that the output of the photo-multiplier drives the motor until the slit is no longer illuminated when the reference signal takes over as a result of relay A being operated. When it is desired to stop the member only in positions corresponding to certain selected slits the others may be covered with opaque material or the circuit shown in Fig. 5 used instead of that in Fig. 4. In this relay contacts a1, a2 are replaced by contacts of relay E which is operated in response to the illumination of photo-multiplier 24 only when one of the selected slits approaches the prism edge. This is effected by completing the circuit of relay E through contacts al of the relay A, the wiper of a stepping switch B and one of a series of switches Sl &c. each corresponding to one of the slits. The switches of the selected slits are closed before starting operations. When the first slit approaches the prism edge relay A operates, thereby operating stepping switch coil B through contacts al. After a short time delay relay D operates to open circuit the stepping switch coil thereby stepping the wiper forward to contact 29, so that if S 1 is closed relay E operates to transfer control of the servomotor from the reference to the photo-multiplier control signal. The member is restarted from this first selected position by depression of switch P until the reference signal has moved the member sufficiently to render relay A inoperative. If switch S1 is open relay E remains inoperative when the reference signal continues to drive the member until the position corresponding to the second slit is approached when the sequence of operations described above is repeated. The sector disc may be replaced by an oscillating mask and to stop the motion of the machine member in the event of failure of the light source a relay in series with the lamp may close normally open contacts in series with contacts a2 (or e2 in the case of Fig. 5) to disconnect the search signal. If the position of a slit is inaccurate the error may be corrected by placing a filter in the path of beam 10 or 11 so that the stable position of the member is a certain amount away from the central position with respect to the prism. In an alternative arrangement the masking member is transparent with opaque strips corresponding to the slits in the opaque member, and a slit, slightly wider than a strip image, is placed in front of the prism edge and/or only a region of the prism surface of this width is rendered reflective. The operation is essentially the same as described with reference to Fig. 4 but relay A must now perform the functions previously ascribed to it when the average illumination falls below a certain level. Specification 699,242, [Group XL (b)], is referred to.